Add XML documentation for Android.Mtp

The `Android.Mtp` namespace contains **231** `To be added` placeholders in source XML documentation (excluding `docs\xml\index.xml`).

Example command:

```powershell
rg -n -i --glob 'Android.Mtp/**' --glob 'ns-Android.Mtp.xml' 'to be added' docs\xml
```

Start by running the provided ripgrep command against docs\xml to enumerate the 231 `To be added` placeholders under Android.Mtp, excluding docs\xml\index.xml. Review the surrounding API declarations and existing XML documentation patterns, then replace the placeholders with accurate documentation for the namespace. Done means the search no longer finds those 231 placeholders in the targeted files.

索引モデルが issue の本文から書いたものです。